This is from the Clarification on the Filioque http://www.catholicculture.org/library/view.cfm?recnum=1176
What is this Trinitarian character that the person of the Holy Spirit brings to the very relationship between the Father and the Son? It is the original role of the Spirit in the economy with regard to the mission and work of the Son. The Father is love in is source (2 Cor 13:13; 1 Jn 4:8.16), the Son is "the Son that he loves" (Col 1:14). So a tradition dating back to St Augustine has seen in the Holy Spirit, through whom "God's love has been poured into our hearts" (Rom 5:5), love as the eternal Gift of the Father to his "beloved Son" (Mk 1:11, 9:7; Lk 20:13; Eph 1:6).11
The divine love which has its origin in the Father reposes in "the Son of his love" in order to exist consubstantially through the Son in the person of the Spirit, the Gift of love. This takes into account the fact that, through love, the Holy Spirit orients the whole life of Jesus towards the Father in the fulfilment of his will. The Father sends his Son (Gal 4:4) when Mary conceives him through the operation of the Holy Spirit (cf. Lk 1:35). The Holy Spirit makes Jesus manifest as Son of the Father by resting upon him at baptism (cf. Lk 3:21-22; Jn 1:33). He drives Jesus into the wilderness (cf. Mk 1:12). Jesus returns ("full of the Holy Spirit" (Lk 4:1). Then he begins his ministry "in the power of the Spirit" (Lk 4:14). He is filled with joy in the Spirit, blessing the Father for his gracious will (cf. Lk 10:21). He chooses his apostles "through the Holy Spirit" (Acts 1:2). He casts out demons by the Spirit of God (Mt 12:28). He offers himself to the Father "through the eternal Spirit" (Heb 9:14). On the Cross he "commits his Spirit" into the Father's hands (Lk 23:46). "In the Spirit" he descended to the dead (cf. 1 Pet 3:19), and by the Spirit he was raised from the dead (cf. Rom 8:11) and "designated Son of God in power" (Rom 1:4).12 This role of the Spirit in the innermost human existence of the Son of God made man derives from an eternal Trinitarian relationship through which the Spirit, in his mystery as Gift of Love, characterises the relation between the Father, as source of love, and his beloved Son.
The original character of the person of the Spirit as eternal Gift of the Father's love for his beloved Son shows that the Spirit, while coming from the Son in his mission, is the one who brings human beings into Christ's filial relationship to his Father, for this relationship finds only in him its Trinitarian character: "God has sent the Spirit of his Son into our hearts, crying Abba!Father!" (Gal 4:6). In the mystery of salvation and in the life of the church, the Spirit, therefore, does much more than prolong the work of the Son. In fact, whatever Christ has instituted — Revelation, the church, the sacraments, the apostolic ministry, and its magisterium — calls for constant invocation (epiclesis) of the Holy Spirit and his action (energeia), so that the love that "never ends" (1 Cor 13:8 ) may be made manifest in the communion of the saints with the life of the Trinity.
 St. Thomas Aquinas, following St. Augustine, writes: "If we say of the Holy Spirit that he dwells in the Son, it is in the way that the love of one who loves reposes in the loved one" (Summa Theologica Ia, q.36, a.2, 4um). This doctrine of the Holy Spirit as love has been harmoniously assumed by St. Gregory Palamas into the Greek theology of the ekporeusis from the Father alone: "The Spirit of the most high Word is like an ineffable love of the Father for this Word ineffably generated. A love which this same Word and beloved Son of the Father entertains (chretai) towards the Father: but insofar as he has the Spirit coming with him (sunproelthonta) from the Father and reposing connaturally in him" (Capita physica XXXVI, PG 150, 1144, D-1145 A).
 Cf. John Paul II, Encyclical Dominum et Vivificantem nn.18-24, AAS LXXVIII, 1986, 826-831. Cf. also Catechism of the Catholic Church, nos. 438, 689 690, 695, 727.